Communication service providers are subject to very strict bandwidth usage spectrum constraints, such as technically mandated specifications and regulations imposed by the Federal Communications Commission (FCC), which currently requires that sideband spillage, namely the amount of energy spillover outside a licensed band of interest, be sharply attenuated (e.g., on the order of 60 dB). While such specifications are readily achievable for traditional forms of modulation such as single-carrier frequency modulation (FM), they are difficult to achieve using more contemporary, digitally based modulation formats, such as QPSK modulation.
Keeping the sidebands attenuated sufficiently to meet industry or regulatory-based requirements using such modulation techniques mandates the use of very linear signal processing systems and components. Although linear components can be implemented at a reasonable cost at relatively low bandwidths (baseband) used in telephone networks, linearizing such components, especially RF power amplifiers, becomes a very costly exercise.
RF power amplifiers are inherently non-linear devices, and generate unwanted intermodulation products (or `intermods`), which manifest themselves as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. This intermodulation distortion is also referred to as spectral regrowth, or spreading of a compact spectrum into spectral regions that do not appear in the RF input signal. The distortion introduced by an RF amplifier causes the phase and amplitude of its amplified output signal to depart from the respective phase and amplitude of the input signal, and may be considered as an incidental (and undesired) AM-to-AM and/or AM-to-PM of the input signal.
One brute force technique for linearizing an RF power amplifier is to build the amplifier as large, high power device and then operate the amplifier at a low power level that is only a small percentage of its rated output power, where the RF amplifier's transfer function is relatively linear. The obvious drawback to this approach is the inefficiency, as well as the high cost and large size. Other prior art attempts to account for RF amplifier degradation have included coupling a `pre-processing` correction loop in the path of the amplifier's input signal, and/or coupling a `post-processing`, feed-forward correction loop with the amplifier's output signal.
The purpose of a preprocessing correction loop is modify the RF amplifier's input signal path. Ideally the control signal causes the signal path adjustment mechanism to produce a signal control characteristic that has been predetermined to be the inverse of the distortion expected at the output of the RF amplifier. As a consequence, when subjected to the transfer function of the RF amplifier, it will optimally effectively cancel the amplifier's anticipated distortion behavior. The mechanism may be made adaptive by extracting the RF error signal component in the output of the RF amplifier and adjusting the control signal in accordance with the such extracted error behavior of the RF amplifier during real time operation, so as to effectively continuously minimize distortion in the amplifier's output.
A post-processing, feed-forward correction loop, on the other hand, serves to extract the amount of RF error (distortion) present in the RF amplifier's output signal, amplify that extracted distortion signal to the proper level, and then reinject the amplified RF error signal at equal amplitude and opposite phase back into a downstream output path of the RF amplifier, such that (ideally) the amplifier distortion is effectively canceled. To extract this error, the output of the RF amplifier is combined in an RF cancellation combiner with the RF input signal (which is used as a reference), so that, ideally, all carrier components (which give rise to the baseband intermods referenced above) are effectively canceled, leaving only the RF error.
In the past, mechanisms to minimize such RF carrier components have involved the use of analog phase and amplitude adjustment circuits, which attempt to align the phase and amplitude of the two RF signals, using differential amplifier and phase detector circuitry to control phase shifter and attenuator elements installed in one or both RF signal paths. A major shortcoming of this conventional approach is the fact that DC offsets of the detector circuits tend to dominate when the energy in the RF signals becomes relatively low, causing the carriers to become misaligned. While this misalignment is not of practical significance for low energy signals, it becomes a major problem when a large RF pulse is received, and the loop does not have sufficient time to respond. When misaligned high energy content RF carriers are applied to the combiner, unwanted RF carrier energy will contribute substantially to the content of the RF error signal, and introduce distortion in the feed-forward loop by overdriving the error amplifier. Also, if the amount of RF carrier energy resulting from misalignment of the two signals is large, the resulting RF error signal can actually damage the error amplifier feeding the downstream feed-forward injection loop.